The present invention relates to new isocyanate prepolymers having predominantly or exclusively blocked isocyanate groups in which at least two different blocking agents are present, a process for their production and their use for the production of flexible epoxy resin systems.
Synthetic resins based on epoxy resins have a number of advantageous properties. Such desirable properties include, for example, good adhesion to organic and inorganic substrates, good solvent resistance and high resistance to chemicals. However, due to their high cross-linking density, amine-cured epoxy resins (particularly those based on diphenylolpropane (bisphenol A) and epichlorohydrin) are often brittle, having glass transition regions above 20.degree. C. Consequently, these synthetic resins do not generally meet the impact strength, shock resistance and high flexibility requirements for many applications. This is particularly true with respect to construction applications where durable bridging of shrinkage cracks (e.g., in concrete) is necessary.
The elasticity of these resins can be increased internally to a certain extent by reducing the cross-linking density and externally by adding softeners.
External elasticizers such as tar, phthalate esters, high-boiling alcohols and vinyl polymers are unreactive and are not incorporated into the duromer network. They bring about expansion solely by occupying space. Internal elasticizing through reduction of the cross-linking density can be achieved by reducing the functionality of the curing agent. Longchain, low-functional aminoamides based on dimerized fatty acids have been successfully used to reduce crosslinking density. However, these aminoamides cannot be employed in all application areas due to insufficient elasticizing.
A good and lasting elasticizing of epoxy resins can be achieved by combination of the resin with a polyurethane. Elasticized plastics composed of epoxy resins, polyfunctional carbamate aryl esters and polyamines are described, for example, in DE-AS 2,152,606, but the plastics thus prepared do not satisfy all of the practical requirements for such materials. More specifically, the carbamate aryl esters described in DE-AS 2,152,606 have a very high viscosity. Therefore, in many cases they have to be processed with added softeners. Further, the mechanical properties of these prior art plastics do not meet the technical requirements for many applications.